Ha! DS1 wanted me to knit a banana the other day.
Seriously, I'd say it was doable. You'll want to use some cardboard inside for shaping (the chimney etc, and the front face), and you'll be doing a bit of intarsia ...
Off the top of my head, looking at a picture of thomas ...
You want to knit the face bit, which is a circle ... you could do it with a pinpoint cast on, then knit a grey circle and embroider on the face? Or you could knit it top-down, but that'd not really be round, but the fact could be intarsia'd on.
Then a black tube (pick up from the edges of circle, or sew together at end, based on your preferences), with a tube going up from there.
The corners are the tricky bit, tbh - Thomas is pretty square. But if you use cardboard or plastic in the filling, then that could be ok ... Oh, and of course it won't be possible to put anyone in the cabin ...